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41 pages, 549 KiB

Open AccessArticle

Comparing a Gauge-Invariant Formulation and a “Conventional Complete Gauge-Fixing Approach” for l=0,1-Mode Perturbations on the Schwarzschild Background Spacetime

by Kouji Nakamura

Universe 2024, 10(11), 420; https://doi.org/10.3390/universe10110420 - 7 Nov 2024

Viewed by 442

Abstract

This article provides a comparison of the gauge-invariant formulation for

l = 0, 1

-mode perturbations on the Schwarzschild background spacetime, proposed by the same author in 2021, and a “conventional complete gauge-fixing approach” where the spherical harmonic functions

Y_{l m}

[...] Read more.

This article provides a comparison of the gauge-invariant formulation for

l = 0, 1

-mode perturbations on the Schwarzschild background spacetime, proposed by the same author in 2021, and a “conventional complete gauge-fixing approach” where the spherical harmonic functions

Y_{l m}

as the scalar harmonics are used from the starting point. Although it is often stated that “gauge-invariant formulations in general-relativistic perturbations are equivalent to complete gauge-fixing approaches”, we conclude that, as a result of this comparison, the derived solutions through the proposed gauge-invariant formulation and those through a “conventional complete gauge-fixing approach” are different. It is pointed out that there is a case where the boundary conditions and initial conditions are restricted in a conventional complete gauge-fixing approach. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

32 pages, 8140 KiB

Open AccessArticle

Constraining the Initial Mass Function via Stellar Transients

by Francesco Gabrielli, Lumen Boco, Giancarlo Ghirlanda, Om Sharan Salafia, Ruben Salvaterra, Mario Spera and Andrea Lapi

Universe 2024, 10(10), 383; https://doi.org/10.3390/universe10100383 - 29 Sep 2024

Viewed by 986

Abstract

The stellar initial mass function (IMF) represents a fundamental quantity in astrophysics and cosmology describing the mass distribution of stars from low mass all the way up to massive and very massive stars. It is intimately linked to a wide variety of topics, [...] Read more.

The stellar initial mass function (IMF) represents a fundamental quantity in astrophysics and cosmology describing the mass distribution of stars from low mass all the way up to massive and very massive stars. It is intimately linked to a wide variety of topics, including stellar and binary evolution, galaxy evolution, chemical enrichment, and cosmological reionization. Nonetheless, the IMF still remains highly uncertain. In this work, we aim to determine the IMF with a novel approach based on the observed rates of transients of stellar origin. We parametrize the IMF with a simple but flexible Larson shape, and insert it into a parametric model for the cosmic UV luminosity density, local stellar mass density, type Ia supernova (SN Ia), core-collapse supernova (CCSN), and long gamma-ray burst (LGRB) rates as a function of redshift. We constrain our free parameters by matching the model predictions to a set of empirical determinations for the corresponding quantities via a Bayesian Markov Chain Monte Carlo method. Remarkably, we are able to provide an independent IMF determination with a characteristic mass

m_{c} = 0 . 10_{- 0.08}^{+ 0.24} M_{⊙}

and high-mass slope

ξ = - 2 . 53_{- 0.27}^{+ 0.24}

that are in accordance with the widely used IMF parameterizations (e.g., Salpeter, Kroupa, Chabrier). Moreover, the adoption of an up-to-date recipe for the cosmic metallicity evolution allows us to constrain the maximum metallicity of LGRB progenitors to

Z_{m a x} = 0 . 12_{- 0.05}^{+ 0.29} Z_{⊙}

. We also find which progenitor fraction actually leads to SN Ia or LGRB emission (e.g., due to binary interaction or jet-launching conditions), put constraints on the CCSN and LGRB progenitor mass ranges, and test the IMF universality. These results show the potential of this kind of approach for studying the IMF, its putative evolution with the galactic environment and cosmic history, and the properties of SN Ia, CCSN, and LGRB progenitors, especially considering the wealth of data incoming in the future. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

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21 pages, 784 KiB

Open AccessArticle

Soft X-ray Spectrum Changes over the 35-Day Cycle in Hercules X-1 Observed with AstroSat SXT

by Denis Leahy and Riddhiman Sharma

Universe 2024, 10(7), 298; https://doi.org/10.3390/universe10070298 - 15 Jul 2024

Viewed by 851

Abstract

Observations of the X-ray binary system Her X-1 by the AstroSat Soft X-ray Telescope (SXT) were carried out in 2020 through 2023 with the goals of measuring X-ray spectrum changes with the 35-day disk precession phase and measuring eclipses at different 35-day phases. [...] Read more.

Observations of the X-ray binary system Her X-1 by the AstroSat Soft X-ray Telescope (SXT) were carried out in 2020 through 2023 with the goals of measuring X-ray spectrum changes with the 35-day disk precession phase and measuring eclipses at different 35-day phases. Her X-1 exhibits a regular flux modulation with a period of ≃35 days with different intensity levels at various 35-day phases (called “states”). The four multi-day long observations were scheduled to cover most of these states. Each 35-day phase was determined using monitoring observations with the Swift Burst Alert Telescope (BAT). Nine eclipses were observed in the range of 35-day phases, with at least one eclipse during each observation. Data with dips were separated from data without dips. The variation in X-ray spectral parameters vs. 35-day phase shows the following: eclipse parameters are nearly constant, showing that the scattering corona does not change with 35-day phase; dips show an increase in covering fraction but not column density compared to non-dip data; the1 keV line normalization behaves similarly to the powerlaw normalization, consistent with an origin near the powerlaw emission region, likely the magnetospheric accretion flow from the inner disk onto the neutron star; and the blackbody normalization (area) is large (

\sim 3 \times 10^{5}

km²) during the Main High and Short High states, consistent with the inner edge of the accretion disk. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

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11 pages, 318 KiB

Open AccessArticle

Thermodynamics of Magnetic Black Holes with Nonlinear Electrodynamics in Extended Phase Space

by Sergey Il’ich Kruglov

Universe 2024, 10(7), 295; https://doi.org/10.3390/universe10070295 - 13 Jul 2024

Viewed by 644

Abstract

We study Einstein’s gravity in AdS space coupled to nonlinear electrodynamics. Thermodynamics in extended phase space of magnetically charged black holes is investigated. We compute the metric and mass functions and their asymptotics, showing that black holes may have one or two horizons. [...] Read more.

We study Einstein’s gravity in AdS space coupled to nonlinear electrodynamics. Thermodynamics in extended phase space of magnetically charged black holes is investigated. We compute the metric and mass functions and their asymptotics, showing that black holes may have one or two horizons. The metric function is regular,

f (0) = 1

, and corrections to the Reissner–Nordström solution are in the order of

O (r^{- 3})

when the Schwarzschild mass is zero. We prove that the first law of black hole thermodynamics and the generalized Smarr relation hold. The magnetic potential and vacuum polarization conjugated to coupling are computed and depicted. We calculate the Gibbs free energy and the heat capacity showing that first-order and second-order phase transitions take place. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

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18 pages, 967 KiB

Open AccessArticle

Gravitating Scalarons with Inverted Higgs Potential

by Xiao Yan Chew and Kok-Geng Lim

Universe 2024, 10(5), 212; https://doi.org/10.3390/universe10050212 - 10 May 2024

Cited by 4 | Viewed by 1190

Abstract

Previously, a class of regular and asymptotically flat gravitating scalar solitons (scalarons) has been constructed in the Einstein–Klein–Gordon (EKG) theory by adopting a phantom field with Higgs-like potential where the kinetic term has the wrong sign and the scalaron possesses the negative Arnowitt–Deser–Misner [...] Read more.

Previously, a class of regular and asymptotically flat gravitating scalar solitons (scalarons) has been constructed in the Einstein–Klein–Gordon (EKG) theory by adopting a phantom field with Higgs-like potential where the kinetic term has the wrong sign and the scalaron possesses the negative Arnowitt–Deser–Misner (ADM) mass as a consequence. In this paper, we demonstrate that the use of the phantom field can be avoided by inverting the Higgs-like potential in the EKG system when the kinetic term has a proper sign, such that the corresponding gravitating scalaron can possess the positive ADM mass. We systematically study the basic properties of the gravitating scalaron, such as the ADM mass, the energy conditions, the geodesics of test particles, etc. Moreover, we find that it can be smoothly connected to the counterpart hairy black hole solutions from our recent work in the small horizon limit. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

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19 pages, 686 KiB

Open AccessArticle

Probing the Propeller Regime with Symbiotic X-ray Binaries

by Marina D. Afonina and Sergei B. Popov

Universe 2024, 10(5), 205; https://doi.org/10.3390/universe10050205 - 3 May 2024

Cited by 2 | Viewed by 977

Abstract

At the moment, there are two neutron star X-ray binaries with massive red supergiants as donors. Recently, De et al. (2023) proposed that the system SWIFT J0850.8-4219 contains a neutron star at the propeller stage. We study this possibility by applying various models [...] Read more.

At the moment, there are two neutron star X-ray binaries with massive red supergiants as donors. Recently, De et al. (2023) proposed that the system SWIFT J0850.8-4219 contains a neutron star at the propeller stage. We study this possibility by applying various models of propeller spin-down. We demonstrate that the duration of the propeller stage is very sensitive to the regime of rotational losses. Only in the case of a relatively slow propeller model proposed by Davies and Pringle in 1981, the duration of the propeller is long enough to provide a significant probability to observe the system at this stage. Future determination of the system parameters (orbital and spin periods, magnetic field of the compact object, etc.) will allow putting strong constraints on the propeller behavior. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

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18 pages, 2427 KiB

Open AccessArticle

Linear Stability Analysis of Relativistic Magnetized Jets: The Minimalist Approach

by Nektarios Vlahakis

Universe 2024, 10(4), 183; https://doi.org/10.3390/universe10040183 - 17 Apr 2024

Cited by 1 | Viewed by 1308

Abstract

A minimalist approach to the linear stability problem in fluid dynamics is developed that ensures efficiency by utilizing only the essential elements required to find the eigenvalues for given boundary conditions. It is shown that the problem is equivalent to a single first-order [...] Read more.

A minimalist approach to the linear stability problem in fluid dynamics is developed that ensures efficiency by utilizing only the essential elements required to find the eigenvalues for given boundary conditions. It is shown that the problem is equivalent to a single first-order ordinary differential equation, and that studying the argument of the unknown complex function in the eigenvalue space is sufficient to find the dispersion relation. The method is applied to a model for relativistic magnetized astrophysical jets. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

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Review

Jump to: Research

34 pages, 7190 KiB

Open AccessReview

Magnetar QPOs and Neutron Star Crust Elasticity

by Hajime Sotani

Universe 2024, 10(6), 231; https://doi.org/10.3390/universe10060231 - 22 May 2024

Cited by 5 | Viewed by 1201

Abstract

The crust region is a tiny fraction of neutron stars, but it has a variety of physical properties and plays an important role in astronomical observations. One of the properties characterizing the crust is elasticity. In this review, with the approach of asteroseismology, [...] Read more.

The crust region is a tiny fraction of neutron stars, but it has a variety of physical properties and plays an important role in astronomical observations. One of the properties characterizing the crust is elasticity. In this review, with the approach of asteroseismology, we systematically examine neutron star oscillations excited by crust elasticity, adopting the Cowling approximation. In particular, by identifying the quasi-periodic oscillations observed in magnetar flares with the torsional oscillations, we make a constraint on the nuclear saturation parameters. In addition, we also discuss how the shear and interface modes depend on the neutron star properties. Once one detects an additional signal associated with neutron star oscillations, one can obtain a more severe constraint on the saturation parameters and/or neutron star properties, which must be a qualitatively different constraint obtained from terrestrial experiments and help us to complementarily understand astrophysics and nuclear physics. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

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